1. Field of the Invention
The present invention relates to a laser device. More specifically, the present invention relates to a monolithic multi-wavelength laser device including a plurality of lasing parts formed on the same substrate.
The present invention also relates to a method of fabricating the aforementioned monolithic multi-wavelength laser device.
2. Description of the Background Art
An optical disk such as a CD (compact disk), a DVD (digital video disk) or an MD (Mini Disc) optically recording information is widely utilized as a large capacity recording medium at present.
The information is recorded in or reproduced from the optical disk through an optical pickup. A laser beam of the 780 nm waveband is employed for an optical pickup reproducing information from a CD while that of the 650 nm waveband is employed for an optical pickup reproducing information from a DVD, in response to the difference in recording density depending on the types of the optical disks.
In recent years, an optical disk capable of reproducing information from both of a CD and a DVD with an optical pickup having a multi-wavelength semiconductor laser device prepared by combining semiconductor laser devices having emission wavelengths of the 650 nm and 780 nm bands with each other has been developed in place of dedicated optical disks for individually reproducing information from the CD and the DVD respectively, as described in Japanese Patent Laying-Open No. 01-204487 or 2000-091698, for example.
FIG. 6 is a schematic sectional view illustrating an exemplary conventional monolithic multi-wavelength laser device. Throughout the accompanying drawings, identical reference numerals denote the same or corresponding portions. Throughout the accompanying drawings, further, dimensions such as lengths, thicknesses and widths are properly changed and not in actual dimensional relation, for the purpose of clarifying and simplifying the illustration.
In the monolithic multi-wavelength laser device shown in FIG. 6, a CD lasing part 602 and a DVD lasing part 603 are parallely formed on an inclined n-type GaAs substrate 601 having a main surface offset by 15° from the crystallographic (001) plane in the [110] orientation.
An n-type GaAs buffer layer 604, an n-type Al0.5Ga0.5 As cladding layer 605, an undoped Al0.3Ga0.7As guide layer 606, an active layer 607, another undoped Al0.3Ga0.7As guide layer 608, a p-type Al0.5Ga0.5As first cladding layer 609 and a p-type GaAs etching stopper layer 610 are successively formed on the CD lasing part 602. A ridge-shaped p-type Al0.5Ga0.5As second cladding layer 611 and a p-type GaAs cap layer 612 are successively formed on the p-type GaAs etching stopper layer 610, and both side surfaces of the ridge-shaped p-type Al0.5Ga0.5As second cladding layer 611 and a p-type GaAs cap layer 612 are filled up with n-type GaAs current blocking layers 613.
On the other hand, an n-type GaAs buffer layer 614, an n-type GaInP buffer layer 615, an n-type (Al0.7Ga0.3)0.5In0.5P cladding layer 616, an undoped (Al0.5Ga0.5)0.5In0.5P guide layer 617, an active layer 618, another undoped (Al0.5Ga0.5)0.5In0.5P guide layer 619, a p-type (Al0.7GaO.3)0.5In0.5 first cladding layer 620 and a p-type GaInP etching stopper layer 621 are successively formed on the DVD lasing part 603. A ridge-shaped p-type (Al0.7Ga0.3)0.5In0.5 second cladding layer 622, a p-type GaInP intermediate layer 623 and a p-type GaAs cap layer 624 are successively formed on the p-type GaInP etching stopper layer 621, and both side surfaces of the ridge-shaped p-type (Al0.7Ga0.3)0.5In0.5 second cladding layer 622, a p-type GaInP intermediate layer 623 and a p-type GaAs cap layer 624 are filled up with n-type GaAs current blocking layers 613.
P-side ohmic electrodes 625 and Mo/Au electrodes 626 are successively formed on the p-type GaAs cap layers 612 and 624 of the CD and DVD lasing parts 602 and 603 respectively. An n-side ohmic electrode 627 is formed on the back surface of the n-type GaAs substrate 601.
A lasing part isolation trench 628 reaching the substrate 601 is formed between the CD and DVD lasing parts 602 and 603, in order to electrically isolate these lasing parts 602 and 603 from each other. Each monolithic multi-wavelength laser device is divided from a wafer along chip dividing trenches 629.
The monolithic multi-wavelength laser device shown in FIG. 6 fabricated in the aforementioned manner is mounted on a submount. At this time, the side closer to the p-side ohmic electrodes 625 is mounted on the surface of the submount while directing that closer to the n-side ohmic electrode 627 upward, and the submount is attached onto a prescribed stem.
When sectionally observed, however, the aforementioned p-side ohmic electrodes 625 have large numbers of gaps around the heads of the ridge portions of the CD and DVD lasing parts 602 and 603, leading to inferior heat dissipativity. Further, the operating current of the monolithic multi-wavelength laser device including a loss guided structure utilizing the current blocking layers 613 of GaAs is increased due to large internal loss, to extremely narrow the tolerance for thermal design etc. in optical pickup design.
In recent years, a technique of growing an epitaxial layer having a lattice constant close to that of an upper cladding layer on the upper cladding layer for chemically stabilizing the upper cladding layer has been developed, as disclosed in Japanese Patent Laying-Open No. 2002-094181, for example. However, this optical confinement structure is still unsatisfactory in performance. Further, a large number of gaps are formed around the heads of ridge portions due to an air ridge structure, disadvantageously leading to inferior heat dissipativity.
While Japanese Patent Laying-Open No. 05-136526 describes a buried (BH) laser prepared by filling up the side surfaces of a waveguide consisting of a double heterostructure with current blocking layers, this buried laser has problems similar to the above, and is not a multi-wavelength laser device.